Hypersonic aircraft may utilize a combination of turbojet engines and RAMjet or SCRAMjet engines. The turbojet engines are used at relatively low speeds and the RAMjet or SCRAMjet engines are used at relatively high speeds. For example, conventional hypersonic aircraft may use turbojets for flight up to approximately Mach 3 and RAMjets or SCRAMjets for flight at higher speeds.
In RAMjet engines, thrust is produced by passing hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. In a RAMjet, high pressure is produced by “ramming” external air into the combustor using the forward speed of the vehicle. SCRAMjet is an acronym for Supersonic Combustion RAMjet. SCRAMjets differ from RAMjets in that combustion takes place at supersonic air velocities through the engine. Since there are no compressors in RAMjets or SCRAMjets, they tend to be lighter and simpler than turbojets, which require a compressor to generate high pressure in the combustor. Since RAMjets and SCRAMjets cannot produce static thrust, other propulsion systems such as turbojet engines must be used to accelerate the vehicle to a speed where the RAMjets or SCRAMjets begin to produce thrust.
RAMjets and SCRAMjets typically include compression ramps at their inlets and expansion ramps at their outlets in order to provide the desired gas pressures entering and leaving the engines. Some hypersonic aircraft engines are equipped with movable inlet ramps. Examples of such movable ramps and their control mechanisms are disclosed in U.S. Pat. Nos. 3,430,640, 4,025,008, 4,307,743, 4,620,679 and 4,991,795, which are incorporated herein by reference.